Hot melt adhesives are used widely in the packaging industry for such applications as case and carton sealing, tray forming, and box forming. The substrates to be bonded include virgin and recycled kraft, high and low density kraft, chipboard, and various types of treated and coated kraft and chipboard. Hot melts are required to exhibit full fiber tearing bonds. This means that all the fiber must be removed from the substrate along the entire length of the adhesive bead when the bond is separated by hand. Generally, in order to get full fiber tearing bonds, hot melts need to be applied at temperatures of about 175.degree. C. The most commonly used application equipment is that which uses a piston pump or gear pump extrusion system. Such equipment is manufactured by Nordson, ITW or Slautterback. Sometimes an adhesive may not give sufficient fiber tearing bonds. In addition to the requirement that the adhesive give full fiber tearing bonds, customers are demanding higher performance in hot melt adhesives such as good thermal stability. Thermal stability generally means that the product will not darken in the glue pot with prolonged aging, will not produce char, skin, or gel, and will not have a substantial viscosity change over time. High application temperatures can cause degradation which leads to char, skin, and gel formation, discoloration, and viscosity changes. Therefore, lowering the application temperature is desirable because it can improve the thermal stability to a large extent. In addition to improving thermal stability, lowering the application temperature increases safety to workers by reducing the risk of burns, decreases the amount of electricity required to heat the adhesives which can result in savings in energy costs, decreases maintenance costs, and reduces the amount of odors and fumes coming from the adhesive. Decreasing the odor and fumes coming from the adhesive can be very appealing to customers, and to the employees who work in the plants which utilize the hot melt adhesives. Hot melt adhesives are typically applied at temperatures of about 175.degree. C. For the aforementioned reasons, it is desirable to apply hot melt adhesives at temperatures of below about 155.degree. C., and preferably about 135.degree. C. to about 150.degree. C. Commercial adhesives designed for application temperatures of about 135.degree. C. to about 155.degree. C. are currently available. These adhesives contain polymers with melt indices of at least 750 g/10 min. It should be recognized that adhesives for lower application temperatures still fall under the general category of hot melt adhesives. In designing products for low temperature application, it is necessary to use lower melting raw materials. This in turn requires that heat resistance is necessarily sacrificed because lower melting raw materials lead to lower melting adhesives which in turn cannot withstand temperatures as high as conventional hot melt packaging adhesives. It is therefore desirable to develop hot melt adhesives that can maintain excellent heat resistance while applied at low application temperatures.
U.S. Pat. No. 5,041,482 to Ornsteen et al. issued Aug. 20, 1991 discloses that glue stick adhesives for use in glue guns can be applied at application temperatures in the 82.degree. C. to 138.degree. C. range, and preferably less than 121.degree. C. Ornsteen teaches the use of backbone polymers that have a melt index in excess of 750 g/10 min., such as ethylene vinyl acetate, polyethylene, and polypropylene. Additionally, glue stick adhesives are higher in viscosity and will not work in piston pump or gear pump extrusion equipment like those manufactured by Nordson, ITW or Slautterback. This type of equipment is used widely in the packaging industry for applying adhesives via an extrusion method. Piston pump or gear pump extrusion equipment requires a much lower viscosity than that which can be used in a glue gun. Therefore, glue sticks would not be used in Nordson type hot melt application equipment.
U.S. Pat. No. 5,373,049 to Ornsteen et al. issued Dec. 13, 1994 teaches cool melt adhesives designed with backbone polymers having a melt index of at least 750 g/10 min. Polymers with melt indices of lower than 650 g/10 min. may be used in combination with said polymer but not as the sole polymer in the formula. In fact, Ornstern discusses that traditional hot melt adhesives have polymers with melt indices in the 1.5 to 550 g/10 min. range making them not amenable for application processes operating at temperatures of below 150.degree. C. due to high viscosities. Ornstern also teaches the use of ethylene vinyl acetate polymers, polyethylene, and polypropylene as backbone polymers and not the use of radial styrene-isoprene-styrene block copolymers which have melt indices of less than 100 g/10 min., and often less than 10 g/min.
U.S. Pat. No. 5,550,472 to Liedermooy et al. issued Mar. 19, 1996 discloses a low application temperature hot melt based on ethylene n-butyl acrylate having a melt index of at least 600; a terpene phenolic tackifying resin; a low melting point synthetic Fischer-Tropsch wax; and a polymer additive of ethylene vinyl acetate, ethylene methyl acrylate, ethylene acrylic acid, polyethylene, polypropylene, poly-(butene-1-co-ethylene), and lower melt index ethylene n-butyl acrylate copolymers. This patent does not teach how to make hot melt adhesives using as the base polymer, a block copolymer having a melt index of less than about 100 g/10 min.
U.S. Pat. No. 5,292,819 to Diehl et al., issued Mar. 8, 1994, discloses and claims the use of a radial styrene-isoprene-styrene (S-I-S) block copolymer having the general configuration (pI-pS-pB).sub.n X where n is a number greater than 2 for use in various hot melt applications such as for disposable articles. Diehl is directed toward pressure sensitive adhesives which are slow setting, and lack the heat resistance required for most packaging applications.
U.S. Pat. No. 5,372,870 to Diehl et al. issued Dec. 13, 1991 also discloses and claims the use of the same radial SIS block copolymer as above, for use in adhesives. Diehl discloses and claims up to 5% based on the weight of the composition of a hydrocarbon wax sufficient to form an adhesive composition useful for lining magazines or books, or for packaging and carton sealing. It is neither suggested nor contemplated to use more than 5% wax. In fact, those compositions exemplified in Diehl comprise 20% of a plasticizer with no wax. These compositions have a slow rate of set and higher viscosities. Furthermore, those compositions exemplified in Diehl are pressure sensitive adhesives which have an indefinite open time.
U.S. Pat. No. 5,399,627 to Diehl et al., issued Mar. 21, 1995 again discloses and claims the block copolymer above for use in hot melt adhesives for tapes. The physical characteristics for a hot melt tape adhesive are vastly different than the physical characteristics required of a hot melt packaging adhesive. Diehl teaches hot melt adhesives that have viscosities of greater than about 25,000 cPs at about 175.degree. C. These adhesives are much too high in viscosity for application temperatures of less than about 155.degree. C., and could not be run on standard packaging equipment which generally requires adhesives having viscosities of about 1,000 cPs at application temperature. Tape adhesives are also pressure sensitive having desirable characteristics such as tack at ambient temperatures which allows the tape to be adhered with hand pressure. Such adhesives contain no wax component. These adhesives would be too slow and would lack the heat resistance required for most packaging applications.
European Pat. Application 0,413,137,A2 published Feb. 20, 1991 teaches the use of a substantially radial styrene-butadiene block copolymer for use in a polyethylene terephthalate bottle assembly. These adhesives are applied at temperatures of at least 175.degree. C. making the viscosities of the products used in the polyethylene terephthalate bottle assembly too high in viscosity for application temperatures of less than about 155.degree. C.
Although block copolymers, in combination with a tackifying resin and a wax have been used in the packaging industry, current products are too high in viscosity lo be applied at low temperatures, requiring temperatures in excess of 175.degree. C., and even in excess of 185.degree. C. This causes severe stability problems leading to char and gel which in turn results in clogged nozzles.
There remains a need in the packaging industry for block copolymer based adhesives that have fast rates of set and can be applied at low temperatures. Improved cold temperature flexibility and resistance, improved heat resistance and superior adhesion to a variety of substrates can be achieved with block copolymer based adhesives having, low viscosities as compared to ethylene copolymer based counterparts.
The present inventor has now discovered a hot melt adhesive utilizing a unique combination of a block copolymer, a tackifying resin, a wax and a plasticizer that can be used in applications where the application temperature is less than about 155.degree. C. and preferably less than about 150.degree. C. and overcomes the aforementioned problems. It is a surprising result of the invention to achieve such low viscosities with block copolymer based adhesives, while maintaining excellent heat resistance and cold temperature flexibility.